How can I get on the same row two post of different categories?

Assuming you’ve already got your HTML/CSS whipped up, you will need two separate custom queries. Here’s one, a here is documentation.

// Custom Query parameters. This one grabs most recent 5 posts from category with ID 9
// For each loop, change "$the_query" variable to something unique
$the_query = new WP_Query( 'cat=9&posts_per_page=5' );
// Begin Custom Loop
while ( $the_query->have_posts() ) : $the_query->the_post();
    // Regular loop stuffs
    echo '<li>';
    the_title();
    echo '</li>';
endwhile;

// Reset Post Data - important for multiple loops on the same page. Should follow each loop.
wp_reset_postdata();